The present invention relates to the compression of gas in a rotary compressor. More particularly, the present invention relates to control of the position of a slide valve in a refrigeration screw compressor by the use of compressor discharge gas sourced from a location where such discharge gas is relatively oil-free and has undergone little or no pressure drop subsequent to its discharge from the compressor's working chamber.
Compressors are used in refrigeration systems to raise the pressure of a refrigerant gas from an evaporator to a condenser pressure (more generically referred to as suction and discharge pressures respectively) which permits the use of the refrigerant to cool a desired medium. Many types of compressors, including rotary screw compressors, are used in such systems. Screw compressors most often employ male and female rotors mounted for rotation in a working chamber which consists of a volume shaped as a pair of parallel intersecting flat-ended cylinders closely toleranced to the exterior dimensions and shapes of the intermeshed screw rotors.
A screw compressor has low and high pressure ends which respectively define suction and discharge ports that open into the working chamber of the compressor. Refrigerant gas at suction pressure enters the suction port from a suction area at the low pressure end of the compressor and is delivered to a chevron-shaped compression pocket defined by the intermeshed rotors and the interior wall of the compressor's working chamber.
As the rotors rotate, the compression pocket is closed off from the suction port and gas compression occurs as the volume of the pocket decreases. The compression pocket is circumferentially and axially displaced to the high pressure end of the compressor by the rotation of the screw rotors and comes into communication with the discharge port. At that point, the now compressed refrigerant gas is discharged from the compressor's working chamber.
Screw compressors most typically employ slide valve arrangements by which the capacity of the compressor is controlled over a continuous operating range. The valve portion of a slide valve assembly is disposed within the rotor housing, which defines the compressor's working chamber, and certain surfaces of the valve portion of the slide valve assembly cooperate in the definition of the working chamber.
Slide valves are most typically axially moveable to expose a portion of the working chamber and the rotors therein to a location within the rotor housing of a screw compressor, other than the suction port, which is at suction pressure. As a slide valve opens to greater and greater degrees, a larger portion of the working chamber and the screw rotors disposed therein are exposed to suction pressure. The portion of the rotors and working chamber so exposed and the chevron shaped pockets they define are incapable of engaging in the compression process and the compressor's capacity is proportionately reduced. The positioning of a slide valve between the extremes of the full load and unload positions is relatively easily controlled as is, therefore, the capacity of both the compressor and the refrigeration system in which the compressor is employed.
Historically, screw compressor slide valves have been positioned hydraulically using oil which has a multiplicity of other uses within such compressors. In refrigeration chiller applications, such other uses include bearing lubrication and the injection of such oil into the working chamber of the compressor for sealing and cooling purposes.
Such oil is most typically sourced from an oil separator downstream of the compressor where discharge pressure is used to drive oil to compressor injection ports and bearing surfaces and to control the position of the compressor's slide valve. It will be noted however, in the context of the present invention, that the pressure in the oil separator will be somewhat reduced from the pressure of the gas as it issues from the compressor's working chamber as a result of the pressure drop the discharge gas will experience in its travel to the oil separator. In any case, however, the pressure differential between the relatively higher pressure source of the oil (the oil separator) and a location within the compressor which is at a relatively lower pressure is taken advantage of to drive oil from the separator to the location of its use in the compressor.
Once used for its intended purpose, such oil is typically vented to or drained from the location of its use to a relatively lower pressure location within the compressor or system in which the compressor is employed. Most commonly, such oil is vented to, drained to or is used, in the first instance, in a location which contains refrigerant gas which is at suction pressure or at some pressure which is intermediate compressor suction and discharge pressure.
Such oil mixes with and becomes entrained in the refrigerant gas which is found in the location to which it is vented, drained or used and is delivered back to the oil separator in the stream of compressed refrigerant gas discharged from the compressor. Such oil, which comprises a relatively large percentage by weight of the gas-oil mixture discharged from the working chamber of a screw compressor, is separated from the refrigerant gas in the oil separator and is deposited in the sump therein. It is then re-directed back to the compressor locations identified above, under the impetus of the pressure in the oil separator for re-use.
Even after the separation process has occurred, oil in the sump of an oil separator will contain refrigerant gas bubbles and/or quantities of dissolved refrigerant. The separated oil may, in fact, contain as much as 10-30% refrigerant by weight depending upon the solubility properties of the particular oil and refrigerant used.
One difficulty and disadvantage in the use of oil sourced from the oil separator to hydraulically position the slide valve in a screw compressor relates to the fact that the oil will, as noted immediately above, typically contain dissolved refrigerant and/or bubbles of refrigerant gas. As a result of the use of such fluid to hydraulically position the piston by which a compressor slide valve is actuated, slide valve response can be inconsistent, erratic and/or slide valve position can drift as dissolved refrigerant entrained in the hydraulic fluid vaporizes (so-called "out-gassing") or as entrained refrigerant gas bubbles collapse.
The out-gassing of refrigerant from the hydraulic fluid, which most often occurs when the pressure in the cylinder in which the slide valve actuating piston is housed is vented to unload the compressor, and/or the collapse of refrigerant gas bubbles entrained in such hydraulic fluid causes a volumetric change in that fluid. That, in turn, affects the ability of the fluid to maintain the slide valve in a desired position or to properly position the slide valve in the first instance.
Still another disadvantage of the use of oil to position the slide valve in a refrigeration screw compressor relates to the fact that the quantity of refrigerant gas bubbles and dissolved liquid refrigerant contained therein varies with time and with the characteristics and composition of the particular batch of lubricant delivered to the slide valve actuating cylinder. In that regard, slide valves are most typically controlled through a supposition that the opening of a load or unload solenoid valve for a predetermined period of time results in the movement of a predetermined volume of hydraulic fluid to or from the slide valve actuating cylinder and slide valve movement that is repeatable and consistent with that period of time. That supposition is, in turn, predicated on the further supposition that the characteristics and composition of the hydraulic fluid directed to or vented from the slide valve actuating cylinder during such a period of time is consistent.
Because of the inconsistency in the characteristics and composition of the fluid supplied to and vented from hydraulically actuated slide valve actuating cylinders with respect to the nature and amount of refrigerant contained therein, slide valve movement during any particular time period may not be precisely consistent, repeatable or predictable. This lack of consistency and repeatability, from the control standpoint, is disadvantageous and reduces the efficiency of the compressor and chiller in which it is employed.
As will be appreciated from the content of U.S. Pat. No. 5,519,273 and U.S. patent application Ser. No. 08/763,775, U.S. Pat. No. 5,832,737, both assigned to the assignee of the present invention and both incorporated herein by reference, arrangements for controlling slide valve position in a screw compressor by the use of a gaseous medium of more uniform consistency rather than a hydraulic medium offer significant advantages. Arrangements are disclosed in that patent and patent application which source gas from one or both of at least two sources of gas within the compressor or the system in which the compressor is employed.
Testing on screw compressors using the arrangements set forth in the above-referenced patent and patent application have suggested that the sourcing of refrigerant gas to actuate the compressors slide valve from the discharge area or plenum, without more and as is taught in both instances, while superior in many respects to hydraulic actuation arrangements, may result in the admission of discharge gas to the slide valve actuating cylinder which contains certain amounts of oil. Excessive oil in such gas makes slide valve control and response more difficult and inconsistent than would be preferred, even though still superior to the consistency of response achieved in hydraulically actuated systems. Further, such arrangements have suggested the need to source gas from at least two rather than a single source of gas at sufficiently high pressure to assure the availability of gas for slide valve actuation purposes under all circumstances within the operating envelope of the chiller in which the compressor is employed. The need for dual gas sources renders such arrangements more complicated and expensive to manufacture and control.
The need therefore exists for an arrangement by which to control the position of a slide valve in a refrigeration screw compressor by the use of a gaseous medium that eliminates the disadvantages associated with the use of hydraulic fluid to do so, that permits more precise and consistent control of the slide valve position, that eliminates moving parts that can, through breakage or wear, lead to loss of or reduced slide valve control and that employs a readily available, single-source of relatively oil-free gas which is reliably at a high enough pressure to ensure that slide valve actuation occurs under the foreseeable operating condition of the refrigeration system in which the compressor is employed.